A roller cone rock bit is the cutting tool used in oil, gas, and mining fields to break through the earth formation to shape a well bore. Load and motion of the bit are transferred to the bearings inside three head and cone assemblies. For the bit where a journal bearing is employed, the main journal bearing is charged with as much as 80 percent of the total radial load. The main journal bearing is composed of the head (as the shaft), the bushing, and the cone (as the housing). This bearing is lubricated and sealed. An outer circumference of the seal is compressed by a gland of the cone so that the seal moves together with the cone and slides against the head (at a sealing surface or seal boss) on the inner circumference of the seal. The seal is thus confined in the seal gland to secure the lubricant within the bearing and prevent debris from invading into the bearing. The longer the seal excludes contamination from the bearing, the longer the bearing life. Therefore, the seal can become the limiter of the rock bit life.
An elastomer seal is known in the prior art as the dominant sealing element in rock bits. Various types of elastomer seals have been developed. The seal is very flexible, and is compatible with the drilling mud. The seal has excellent resilience at relatively high temperatures. Thus, the seal has proved to be sufficient to provide enough sealing force to separate the mud and debris environment from the lubricant over an acceptable period of time.
However, friction between the seal and surfaces of the seal gland as the cone rotates can cause damage to the seal itself. Over time, this damage accumulates to the point where the seal itself fails. Following seal failure, the bearing experiences grease starvation in the contact zone due to loss of lubricant in the bearing system. Thereafter, excessive wear appears on the bearing system surfaces due to shearing and heating caused by sliding friction. The end result is typically scoring, scuffing, and even catastrophic failure like galling or seizure. It is thus imperative that lubrication be retained between contact interface surfaces of the journal bearing. Maintaining seal life is thus critical to maintaining bit life.
One way to extend seal life is to reduce the friction between seal and head. Under typical running conditions, the seal experiences mixed lubrication. In this lubrication regime, more lubricant is necessary at the contact point between the seal and one or more of the gland surfaces in order to reduce the friction. Thus, there is a need in the art to introduce more lubricant in the sealing zone.
Reference is made to FIG. 1 which illustrates a partially broken away view of a typical roller cone rock bit. FIG. 1 more specifically illustrates one head and cone assembly. The general configuration and operation of such a bit is well known to those skilled in the art.
The head 1 of the bit includes the bearing shaft 2. A cutting cone 3 is rotatably positioned on the bearing shaft 2 which functions as a journal. A body portion of the bit includes an upper threaded portion forming a tool joint connection 4 which facilitates connection of the bit to a drill string (not shown). A lubrication system 6 is included to provide lubrication to, and retain lubricant in, the journal bearing between the cone 3 and the bearing shaft 2. This system 6 has a configuration and operation well known to those skilled in the art.
A number of bearing systems are provided in connection with the journal bearing supporting rotation of the cone 3 about the bearing shaft 2. These bearing systems include a first cylindrical friction bearing 10 (also referred to as the main journal bearing herein), ball bearings 12, second cylindrical friction bearing 14, first radial friction (thrust) bearing 16 and second radial friction (thrust) bearing 18.
FIG. 2 illustrates a partially broken away view of FIG. 1 showing the bearing system and sealing system in greater detail. The first cylindrical friction bearing 10 is defined by an outer cylindrical surface 20 on the bearing shaft 2 and an inner cylindrical surface 22 of a bushing 24 which has been press fit into the cone 3. This bushing 24 is a ring-shaped structure typically made of beryllium copper, although the use of other materials is known in the art. The ball bearings 12 ride in an annular raceway 26 defined at the interface between the bearing shaft 2 and cone 3. The second cylindrical friction bearing 14 is defined by an outer cylindrical surface 30 on the bearing shaft 2 and an inner cylindrical surface 32 on the cone 3. The outer cylindrical surface 30 is inwardly radially offset from the outer cylindrical surface 20. The first radial friction bearing 16 is defined between the first and second cylindrical friction bearings 10 and 12 by a first radial surface 40 on the bearing shaft 2 and a second radial surface 42 on the cone 3. The second radial friction bearing 18 is adjacent the second cylindrical friction bearing 12 at the axis of rotation for the cone and is defined by a third radial surface 50 on the bearing shaft 2 and a fourth radial surface 52 on the cone 3.
With respect to the sealing system, an o-ring seal 60 is positioned between cutter cone 3 and the bearing shaft 2. A sealing surface, for example, a cylindrical surface seal boss 62, is provided on the bearing shaft. In the illustrated configuration, this sealing surface provided by the seal boss 62 is cylindrical and outwardly radially offset (by the thickness of the bushing 24) from the outer cylindrical surface 20 of the first friction bearing 10. It will be understood that the sealing surface (of the seal boss 62 for example) could exhibit no offset with respect to the main journal bearing surface, or be inwardly radially offset, if desired. Additionally, it will be understood that the sealing surface (62) need not be cylindrical but rather may be conical if desired. An annular groove is formed in the cone 3 to define the seal gland 64. The groove and sealing surface (seal boss 62) align with each other when the cutting cone 3 is rotatably positioned on the bearing shaft to define the gland 64 region. The o-ring seal 60 is compressed between the surface(s) of the gland 64 and the sealing surface (seal boss 62), and functions to retain lubricant in the bearing area around the bearing systems and prevents any materials (drilling mud and debris) in the well bore from entering into the bearing area.
Load in the bearing system is supported by both asperity contact and hydrodynamic pressure. Lubricant is provided in the first cylindrical friction bearing 10, second cylindrical friction bearing 14, first radial friction bearing 16 and second radial friction bearing 18 between the implicated cylindrical and radial surfaces using the system 6. Lubricant is retained in the bearing system by the compressed seal 60 in the gland 64. That lubricant not only lubricates the bearing system, but also provides a measure of lubricant on the surfaces of the seal gland 64, and especially the sealing surface such as the seal boss 62 surface, that assists in allowing the compressed seal 60 to slide along the sealing surface (for example, seal boss 62 outer cylindrical surface) as the cone rotates.
The seal is designed to withstand a high pressure in downhole drilling applications. That high pressure, together with a designed high compression rate of the seal in gland, compresses the seal tightly against the seal boss 62. The lubricant which is present in the sealing zone at the seal boss surface provides lubrication to the seal and takes away friction heat. In the case where the seal is not well lubricated, it slides dry against the seal boss and a large amount of friction heat is generated. This friction heat is known to be the root cause of seal failure. It is accordingly desirable to introduce more lubricant underneath the seal, such as on the seal boss 62 surface (or other sliding gland surface), in order to reduce friction and carry away heat.
It is not unusual for the bearing to experience grease starvation in these surface contact zones of the bearing system. This can result in scoring, scuffing, and even catastrophic failure like galling or seizure of the journal bearing. There is accordingly also a need to retain lubricant in position trapped between the implicated and opposed cylindrical and radial surfaces of the bearing system.
Reference is made to the following prior art documents: U.S. Pat. No. 3,839,774 (Oct. 8, 1974), U.S. Pat. No. 4,248,485 (Feb. 3, 1981) and U.S. Pat. No. 5,485,890 (Jan. 23, 1996): U.S. Publication 2005/0252691 (Nov. 17, 2005); and PCT Publication WO 2007/146276 (Dec. 21, 2007), the disclosures of which are hereby incorporated by reference.